In the field of outdoor building materials, coated steel sheet having a colored organic film at its outermost layer is being widely used. A typical coated steel sheet is comprised of plated steel sheet treated on its surface for the purpose of imparting adhesion and covered by an organic film of a two-layer structure comprised of a primer coat and a top coating layer. The coated steel sheet has aesthetic properties while having workability enabling it to be able to withstand roll forming or bending etc. and is considered to require excellent corrosion resistance enabling it to withstand long term outdoor exposure.
Coated steel sheet (precoated steel sheet) is steel sheet precoated by the steel sheet manufacturer for use as a material to be worked. It is later cut and shaped by the end user. For this reason, a high coat adhesion able to withstand later post-processing is considered required. Further, the sheet is often used with the cut end face parts bared as they are. In coated steel sheet used in the field of outdoor building materials, the particularly important performance is the corrosion resistance under overhangs. The areas under overhangs are prevented from being cleaned by rain, so particles of sea salt and other corrosive factors borne by the air and deposited there remain there for long periods without being washed away. As a result, the corrosive environment under overhangs is more severe than usual and is known to increase corrosion under the coating called “edge creep” resulting from corrosion from the cut end face parts of the coated steel sheet. That is, overhang corrosion resistance preventing such corrosion has been considered necessary.
To impart corrosion resistance, a coating composition containing hexavalent chrome, called “chromate treatment”, excellent in adhesion and corrosion resistance as surface treatment at coated steel sheet and containing a chrome-based rust-proofing pigment as a primer has been used since the past and is still being broadly used even at the present. However, due to the issue of the environmental load of hexavalent chrome, in recent years, chromate-free coated steel sheet has been strongly demanded in the field of outdoor building materials.
As the surface treatment agent for coated steel sheet, for example, PLT 1 discloses a surface treatment agent containing a silane coupling agent and its hydrolyzed condensates, aqueous dispersed silica, a zirconium compound, etc. in specific ratios. Further, PLT 2 discloses a surface treatment agent containing a silane coupling agent, cationic urethane resin, Zr compound and/or Ti compound, and a fluorine-containing inorganic compound in specific ratios. However, these surface treatment agents are applied to coated steel sheet based on hot dip galvanized steel sheet in the field of household electrical appliances. The ratios of mixture of the compounds and the types or structures of the resins used for obtaining a level of corrosion resistance demanded in the field of outdoor building materials have not been prescribed yet, so sufficient overhang corrosion resistance cannot be obtained by just application of these to coated steel sheet in the field of outdoor building materials.
PLT 3 discloses metal surface treatment compositions containing specific structures of a polyurethane resin, phosphoric acid compound, vanadium compound, zirconium compound, titanium compound, cobalt compound, nickel compound, silane coupling agent, silica particles, etc. However, these treatment compositions are used for temporary rust-proofing of metal materials before coating or without coating. When used instead for surface treatment of coated steel sheet where the mechanism of corrosion differs, the action of elution of the rust-proofing agent contained conversely becomes a cause of blistering of the coating layer, so a sufficient corrosion resistance of the coated steel sheet cannot be obtained. Further, the adhesion of this treatment composition is of a level envisioning the case of coating after processing. The level of working adhesion demanded for coated steel sheet also cannot be obtained.
PLT 4 discloses coated Zn—Al-based alloy plated steel sheet excellent in corrosion resistance having a Zn—Al-based alloy plating layer on which an undercoat in which a magnesium salt and phosphoric acid salt are mixed is formed via a chemical conversion film containing a titanium compound, zirconium compound, fluoride, etc. However, the chemical conversion film in mainly a fluoro compound or other inorganic ingredient. In stringent processing, the chemical conversion film cannot follow the processing and a sufficient coating adhesion cannot be obtained. Even if that chemical conversion film contains an organic resin, a sufficient working adhesion cannot be obtained.